Digital Subscriber Line (DSL) is a modem technology that enables broadband digital data to be transmitted over twisted-pair wire. This is the type of infrastructure that links most home and small business subscribers to their telephone service providers. DSL modems enable users to access digital networks at speeds tens to hundreds of times faster than current analog modems and basic ISDN service. DSL thus opens the most critical bottleneck in local-loop access to high-speed networks, such as Asynchronous Transfer Mode (ATM) and Internet Protocol (IP) networks, without requiring major investments in new infrastructure. DSL systems use special line signals that are well-adapted to the characteristics of twisted-pair wire and to the noise that normally exists on telephone lines.
A range of DSL standards have been defined, known generically as “xDSL,” wherein the various standards have different data rates and other associated features but share common principles of operation. These standards include High-Speed DSL (HDSL), at relative low frequencies (<500 kHz); Asymmetric DSL (ADSL), with an intermediate frequency range (30∝1100 kHz); and Very High Speed DSL (VDSL), in a high frequency range (0.3-20 MHz). VDSL modems support the highest possible bit rates on existing twisted-pair wire. Downstream bit rates of up to 50Mb/sec support such demanding services as video on demand. It is expected that in the near future, VDSL systems will come to dominate subscriber equipment on the telephone network.
One problem of VDSL implementation is that the frequency band of VDSL (up to 20 MHz) overlaps with several RF bands used for amateur radio and radio broadcast transmissions. To avoid interference between VDSL and RF systems, new standards propose that VDSL systems not transmit in RF bands assigned for radio use. But to avoid interference from existing RF systems, VDSL modems must be designed with reduced sensitivity to RF signals. Twisted-pair wire is particularly prone to picking up interference signals from external RF sources. This problem becomes more severe the higher the transmission frequency and can cause significant degradation of VDSL signals.
One method known in the art for rejection of RF interference signals is to use a common mode choke in the twisted-pair telephone line. The common mode choke attenuates RF noise by about 30 dB, but this attenuation is not enough for long cables, wherein the VDSL line signal is typically very small and the RF noise may be large.
Another method for decreasing sensitivity to RF interference is noise cancellation, as described, for example, in PCT Patent Application PCT/US97/06381, published as WO 97/40587, whose disclosure is incorporated herein by reference. This application describes a receiver system for high-speed data communications, such as ADSL or VDSL, having a RF noise canceller. The noise canceller adaptively estimates the radio frequency noise coming into the receiver through twisted pair input lines. The estimate is used to generate a noise cancellation signal, which is subtracted from the signals coming into the receiver. The noise estimate is based on a common-mode reference noise signal, which is sampled at a transformer that couples the input lines to the receiver. The preferred source of the common-mode signal is from a center tap on the input side of the transformer, taken with reference to a chassis ground. It is noted that the common-mode signal could alternatively be obtained from one of the input lines or from the sum of the lines with respect to ground.
The technical solution proposed by PCT/US97/06381 has several disadvantages which make its practical realization difficult. Existing communication standards require that the primary winding of the line transformer be isolated from chassis ground and from the secondary winding. The breakdown voltage of this isolation must be at least 1500 VAC. Therefore, a common mode reference noise signal from the center tap of the primary winding cannot be connected directly to the RF noise canceller, and an additional high-voltage transformer is required between the center tap of the line transformer and the RF noise canceller. A further difficulty with this solution is that it allows RF common mode noise to reach the primary winding of the line transformer without any attenuation. Because of inter-winding capacitance of the transformer, a strong noise signal arises on the secondary winding of transformer, as well. This noise signal can typically be cancelled by appropriate noise cancellation. But if a digital noise canceller is to be used (generally the most practical solution), the high input noise level makes it necessary to use a costly analog/digital (A/D) converter with high dynamic range.